Eh? What's going on here?

I've saved quite a few battles involving 20 to 80 pictish elite berserkers and royal bodyguards destroying entire armies of mercians and such like. I tend to use AI stupidities as much as possible to prevent being flanked. If you can get berserkers into 1 on 1s they are practically invincible, but if they are flanked the kill ratio goes down from around 1:10^500 to 1:10.